Method of checking for the presence of connection balls

ABSTRACT

Method of checking for the presence of connection balls of components, in particular of ball grid arrays by projecting light obliquely onto the components to create reflections and shadows at the connection balls and any impurities, detecting the reflections and shadows and evaluating the detected shadows and reflections to determine which are from connection balls and which are from impurities.

BACKGROUND OF THE INVENTION

The invention relates to a method of checking for the presence ofconnection balls of components by illuminating the component with theconnection balls and detecting reflections and shadows being produced.

In the course of automatically populating substrates, in particularprinted circuit boards or ceramic substrates, with SMD components forsurface mounting (SMD=Surface Mounted Devices), the components, beforebeing emplaced, are visually inspected with regard to their position andthe presence of all the connections, in particular all the connectionballs in the case of ball grid arrays (BGAs), in order subsequently tocorrect the position of the component prior to emplacement on thesubstrate, or to sort out the component owing to missing connections.

WO 96/21343 discloses an apparatus for identifying the position of theconnections of components which uses illumination with light which isobliquely incident on all sides, in order to automatically evaluate thereflections from connection balls and to determine the position of thecomponent so that positional correction can subsequently be performed.In this case, use is made of segmented ring light illumination in orderto illuminate all the connection balls uniformly from different sides.

However, this method does not make it possible to determineunambiguously whether all the connection balls are present at theintended contact points. Soldering residues from the fixing of the ballscan also cause a reflection which resembles a reflection from aconnection ball and simulates a ball that is present. The greatdependence of the reflection position on the surface condition of theconnection balls additionally makes the evaluation more difficult.

T. Ross et al: Inspection technique for solder reflow pad height/volume,IBM Technical Disclosure Bulletin, volume 22, No. 9, February 1980, page4068, discloses a method of determining the height or the volume ofconnection balls on components, where an image-evaluating unitdetermines the shadow area cast by the connection balls and resultingfrom obliquely incident illumination. In this case, the illumination iseffected only on one side, since if the illumination were on all sides,shadow areas cast by the connection balls and capable of evaluationwould not be produced. Dark impurities on the components of the samesize as shadow areas lead to misinterpretations concerning the presenceof connection balls in the case of this method.

SUMMARY OF THE INVENTION

The object of the invention consists in developing a method of checking,without errors, for the presence of connection balls on components, inparticular ball grid arrays (BGAs), with account especially being takenof the erroneous reflections from impurities (soldering residues) on thelocations nominally occupied by connection balls.

The object is achieved according to the invention by means of a methodhaving the steps of illuminating the connection balls with an obliquelyincident light from one direction, detecting both light reflections andshadows cast laterally and evaluating both the reflections and theshadows to determine the presence of a connection ball.

With the connection balls being illuminated on one side by parallellight or light emitted from a point light source, the resulting shadowsand the light reflections from the connection balls are evaluated. Tothat end, the shadow sizes which are dependent on the illuminationdirection and the size of the connection balls are to be determinedtheoretically or experimentally and compared with the instantaneouslymeasured shadow sizes in each case. As a result of this, the influenceof the surface condition of the connection balls on the evaluation isadditionally avoided, in an advantageous manner. As a result of theillumination on one side, a shadow that can be evaluated for the purposeof checking for the presence of the connection balls is advantageouslyproduced, by comparison with the method for measuring the lightreflections which is disclosed in the prior art.

A further advantage of the method consists in the possibility ofretrofitting existing evaluation devices for the detection of the lightreflection, the illumination device and the image-evaluating unit beingadopted. The image analysis is then adapted correspondingly to the novelmethod.

By detecting the shadow and reflection, not only soldering residueswhich cause misinterpretable reflections but also dark impurities of thesame size as the shadow of connection balls are not identifiedincorrectly as connection balls that are present. This ensureserror-free checking for the presence of the connection balls,independently of the surface condition of said balls.

A configuration according to the invention is elucidated in more detailin the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for checking for the presence of connectionballs, in longitudinal section, and

FIG. 2 shows a plan view of a laterally illuminated component with theshadows cast by the connection balls.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 diagrammatically shows, in longitudinal section, how a component,here a ball grid array 1, with a connection ball 2, a metallic impurity3 and also a dark impurity 4 of the same size as the shadow of aconnection ball is illuminated laterally by a light source 5. Lateralillumination with parallel light can also be used in the method. It isparticularly advantageous to use segmented ring light illumination whosesegments can be controlled individually in terms of their intensity,thereby obtaining illumination by light which is obliquely incident onone side.

The connection ball 2 produces a reflection 6 and a shadow 7, and themetallic impurity 3 produces a further reflection 11 and a furthershadow 12, which—in the same way as a dark impurity 4 of the same sizeas the shadow of a connection ball—are imaged via an objective 14 ontoan image-evaluating unit 15, for example a CCD camera. The size of thefurther shadow 12 of the metallic impurity 3 differs from that of theshadow of a connection ball that was intended at the location of themetallic impurity, which, with the aid of the image-evaluating unit 15,allows the missing contact to be identified and the component to besorted out. The dark impurity 4 of the same size as the shadow of aconnection ball can be identified by the absence of an additionalreflection associated with the dark impurity 4, with the aid of theimage-evaluating unit 15.

FIG. 2 illustrates the laterally illuminated component 1 with aplurality of connection balls 2 with the associated shadows 7 and alsothe metallic impurity 3 with the associated further shadows 12 from thedirection of the image-evaluating unit 15. The further shadow 12 of themetallic impurity 4 differs from an expected shadow cast by a connectionball, which is identified by the image-evaluating unit 15.

What is claimed is:
 1. A method of checking for the presence ofconnection balls on components, including ball grid arrays byilluminating the connection balls by light which is obliquely incidentfrom one direction, detecting light reflections from the connectionballs by an image-evaluating unit connected downstream of an imagingoptical arrangement and evaluating a shadow cast laterally with respectto the connection balls and resulting due to the obliquely incidentlight by the image-evaluating unit the improvement comprising using thesize of the shadow to be expected given a specific size of theconnection ball, in conjunction with the presence of the associatedlight reflection from the connection ball as a criterion forautomatically checking for the presence of connection balls.